1. Field of hhe Invention:
The present invention relates to a digital signal transmission system, and particularly to a digital signal synchronizing system which converts an operation speed (bit rate) of given digital signals to be transmitted into an operation speed (bit rate) of a predetermined transmission system (which includes repeaters and the like).
2. Description of the Prior Art:
A conventional digital signal transmission system has been so designed that the bit rate of digital signals to be transmitted is in agreement with the bit rate of the transmission system. When the two bit rates are not the same, therefore, it is not allowed to directly transmit the digital signals.
It is therefore necessary to provide a signal format converter at the sending terminal and at the receiving terminal to establish synchronism with respect to the bit rate of the transmission system. The signal format converter requires a buffer circuit which makes it very difficult to input (write) and output (read) the signals continuously, i.e., in real time. Therefore, the system which is provided with the buffer circuit and which synchronizes the bit rate is limited to a start-stop transmission system which deals with particular digital signals only.
Moreover, it is impossible to multiplex a plurality of asynchronous signals.
Therefore, it is accepted practice to set the bit rate of the transmission system to be in agreement with the bit rate of digital signals that are to be transmitted, or to set the bit rate of digital signals that are to be transmitted to be in agreement with the bit rate ff the transmission system.
Accompanying the development in the digital signal processing technology, however, the number of sending terminals or channels has increased to generate digital signals of many kinds having different bit rates. At the present time where a high-speed transmission is made possible by using a transmission medium of a very broad band such as an optical fiber, furthermore, it will be very advantageous to economically utilize the transmission line if signals having a given bit rate can be transmitted irrespective of the bit rate of the transmission system. Moreover, if these signals could be transmitted as multiplexed signal, the easiness for using the transmission line can be strikingly improved.
The inventor of the present application has already proposed a digital signal transmission system which transmits given digital signals having a bit rate smaller than the bit rate of a transmission system in synchronism with the transmission system in real time (U.S. Pat. No. 4,330,856). This digital transmission system transmits input digital signals having any bit rate by converting them, via a signal format converter, into digital signals having a bit rate higher than the above-mentioned bit rate. Here, the signal format converter comprises a buffer circuit of a small capacity, a circuit which writes onto the buffer circuit at the bit rate of the input digital signals and reads at the bit rate of the transmission system, a circuit which distinguishes the signals that are read out as mark, space or blank, and a coding equipment which converts the mark, space or blank into pulse signals that can be identified with a time slot length of the transmission system or an integer number of times thereof as a reference, depending upon the level of the pulses or the number (run length) of the continuing pulses. That is, as shown in FIG. 1, "1" is converted into "11" or "00", "0" is converted into "1" or "0", and "empty" is converted into "111" or "000". Therefore, when "1" is generated continuously, the efficiency becomes 50%. When "1" and "0" are generated equally, the efficiency becomes about 75%. In this case, however, the system is used at an efficiency of about 60% by taking the safety coefficient into consideration against the overflow. With this system, therefore, the conversion efficiency is as low as about 50 to 60%.
Further, a so-called pulse stuffing system has been known to obtain the conversion efficiency which is close to 100%. According to the pulse stuffing system as shown in FIG. 2, when the original digital signals (a) are to be converted into digital signals (b) having a slightly high bit rate, the difference in the bit rate is adjusted using a stuff pulse St. The position of the stuff pulse St is indicated by a stuff indicating pulse (c). Usually, the digital signals (b) and the stuff indicating pulse (c) are transmitted while being synthesized together. However, when the bit rate is too different between the original digital signals and the digital signals having a high bit rate, the stuff indicating pulse (c) is generated frequently, wnereby the conversion efficiency decreases and it becomes difficult to distinguish the digital signals (b) over the stuff indicating pulses (c). Therefore, the pulse stuffing system can synchronize the digital signals only when they have bit rates that are very close to each other.